Hi Ciaran,
I updated a site to J4 that uses Stella along with 3 extensions (ThumbsGallery, LayerSlideshow and ImageHover) and got the above error. I bought the J4 version of Stella and uploaded the extensions from that package. From googling I learn that it's a problem with one of the extensions. I successfully migrated to J4 another site that uses Beaumont and it went fine. I'm nervous now because I have several sites built with Stella. This site is in French and English. Can it be a problem with the French language package?

Also, in what order does one do the update -- upload J4 versions of template and extensions and then update Joomla? Or the reverse? Does it matter?

Anxiously awaiting a reply...

Kim